Alpha 1-antitrypsin (aaT) is a protease inhibitor of the serpin superfamily. It is the main protein of the alpha-globulin 1 region in gel electrophoresis of plasma, and is also referred to as alpha-1 proteinase inhibitor because it inhibits a wide variety of proteases. Physiologically, it protects tissues from the enzymes of inflammatory cells, especially neutrophil elastase, and its concentration can rise several-fold during acute inflammation. In the absence of aaT, neutrophil elastase is free to break down elastin, an important component in maintaining the elasticity of the lungs, which results in respiratory complications such as emphysema or chronic obstructive pulmonary disease (COPD).
Alpha-1 antitrypsin deficiency is an inherited disorder affecting about 1 in 1,500 to 3,500 individuals of European ancestry, but is uncommon in people of Asian descent. People with alpha-1 antitrypsin deficiency usually develop the first symptoms of lung disease between the ages of 20 and 50, which may present as shortness of breath, reduced ability to exercise, or wheezing, and propensity to respiratory infections. Persons with alpha-1 antitrypsin deficiency often develop emphysema, which can be accelerated by smoking or exposure to tobacco.
Because alpha-1 antitrypsin is expressed in the liver, certain mutations in the encoding SERPINA1 gene can cause misfolding and impaired secretion of alpha-1 antitrypsin, which can lead to liver cirrhosis. About 10 percent of children and 15 percent of adults with alpha-1 antitrypsin deficiency develop cirrhosis due to the formation of scar tissue in the liver and are also at risk for developing a type of liver cancer called hepatocellular carcinoma.
Patients with lung-forms of disease due to alpha-1 antitrypsin deficiency may receive intravenous infusions of alpha-1 antitrypsin, derived from human plasma, which must be administered once weekly. For example, the commercial product Prolastin®C is indicated for chronic augmentation and maintenance therapy in adults with emphysema due to deficiency of alpha1-antitrypsin. Prolastin®C is a plasma-derived formulation of alpha-1-anti-trypsin (a.k.a. alpha 1-antiproteinase) that, in treatment of congenital aaT deficiency, has a recommended dose regiment of 60 mg/kg once per week; a requirement of more than 200 grams of protein/patient/year. Because it is derived from plasma, the drug supply is limited, with only about 5% of the estimated 170,000 eligible patients in the United States and the European Union receiving therapy. Native glycosylation is critical for maintaining sufficient half-life of aaT, such that unmodified forms of recombinant forms of alpha-1 antitrypsin have, with their documented short half-life thus far been unsuccessful solutions to the supply and dosing schedule problems. (Travis et al., JBC (1985) 260:4384-4389; Casolaro, M, et al., J. Appl. Physiol. (1987) 63:2015-2023).
Both because the currently approved source of aaT is limited to plasma and because of the frequency of dosing required, there remains a need for a recombinant form of the protein with a longer terminal half-life that can be administered on a schedule that is at least similar or is improved compared to the plasma-derived product. Chemical modifications to a therapeutic protein can modify its in vivo clearance rate and subsequent half-life. One example of a common modification is the addition of a polyethylene glycol (PEG) moiety, typically coupled to the protein via an aldehyde or N-hydroxysuccinimide (NHS) group on the PEG reacting with an amine group (e.g. lysine side chain or the N-terminus). However, the conjugation step can result in the formation of heterogeneous product mixtures that need to be separated, leading to significant product loss and complexity of manufacturing and does not result in a completely chemically-uniform product. Also, the pharmacologic function of pharmacologically-active proteins may be hampered if amino acid side chains in the vicinity of its binding site become modified by the PEGylation process. Other approaches include the genetic fusion of an Fc domain to the therapeutic protein, which increases the size of the therapeutic protein, hence reducing the rate of clearance through the kidney. Additionally, the Fc domain confers the ability to bind to, and be recycled from lysosomes by, the FcRn receptor, which results in increased pharmacokinetic half-life. Unfortunately, the Fc domain does not fold efficiently during recombinant expression, and tends to form insoluble precipitates known as inclusion bodies. These inclusion bodies must be solubilized and functional protein must be renatured from the misfolded aggregate, a time-consuming, inefficient, and expensive process. Accordingly, there remains a need for alpha-1 antitrypsin compositions and formulations with increased half-life and retention of activity and bioavailability when administered intravenously, subcutaneously, intramuscularly, or by the pulmonary route as part of a preventive and/or therapeutic regimen for alpha-1 antitrypsin deficiency that can be administered less frequently, and are safer and less complicated and costly to produce.